High frequency amplifier



May 7, 1940. H. c. ALLEN ET AL HIGH FREQUENCY AMPLIFIER 2 Sheets-Sheet 1 Filed March 9, 1957 Hfifi. .w w 56 $1 1111 a 2 z M ii 8 Z Illlll' W W 2 8 ZW/ IIIJUII lllll 152 ii 2 g 0 i a H5 0. n r m f. a

May 7, 1940. H. c. ALLEN a- AL.

HIGH FREQUENCY AMPLIFIER Filed March 9, 1937 2 Sheets-Sheet 2 Patented May 7, 1940 UNITED] STATES HIGH FREQUENCY AMPLIFIER.

Horace C. Allen, Marlton, and Gustave L. Grundmann, Westm'o'nt, N. J assignors to Radio Corporation of America, a corporation of Delaware Application March 9, 1937, Serial No. 129,804.

4 Claims.

Our invention relates to high frequency amplifiers, and particularly to intermediate fre quency amplifiers of the band pass type for television receivers or the like.

An object of our invention is to provide a band pass filter unit in which the coupling between the primary circuit and the secondary circuit can be held to the required limits by production methods.

A further object of our invention is to provide an improved intermediate frequency amplifier unit which is compact and which is so designed that each amplifier stage'is a single unit which may be tested and adjusted before being mounted on the receiver chassis.

A further object of our invention is to provide an improved amplifier circuit in which a minimum number of filter elements are required.

In a preferred embodiment of our invention, the band pass filter for coupling two electric discharge tubes includes two coupled tuned circuits,

each of these circuits being tuned to a variable inductance coil having a movable magnetic core and the two circuits being coupled either by means of an inductance coil or by means of a condenser common to the two tuned circuits.

The tuned circuit elements and one amplifier tube are mounted upon a sub-base, this sub-base also having mounted thereon the necessary filter resistors and filter condensers. Improved filtering to prevent intermediate frequency feed-backmay be obtained by connecting the grid resistor of an amplifier unit to the low potential end of the tuned secondary coil instead of to the grid end of this coil.

Other objects, features and advantages of our invention will appear from the following description, taken in connection with the accompanying drawings, in which Figure 1 is a side view, partly in section, of an intermediate amplifier stage, constructed in accordance with one embodiment of our invention,

Figure 2 is an end view, partly in section, of the amplifier unit shown in Figure 1,

Figure 3 is a circuit diagram of the amplifier unit shown in Figures 1 and 2.

The mechanical construction of our improved intermediate frequency amplifier stage will be understood by referring to Figs. 1 and 2. As shown in Fig. 1, an electric discharge tube socket l is mounted on the under side of a sub-base 2 and underneath an opening therein whereby an amplifier tube, indicated at 3, may be mounted.-

on the upper side of the sub-base. Unless a metal tube is employed, a support 4 for a shielding can is mounted on the upper side of the subbase 2, whereby a shielding can, indicated at 6, may be positioned over the electric discharge tube 3. The particular tube illustrated is of the pentically positioned tubes 8 and 9 of fiber or the like, upon which are wound the primary and secondary coils H and I2, respectively. An additional coil I3 is provided, which preferably is wound on the same tube as the primary coil II, this coil being employed, as will be described hereinafter, for the purpose of providing the necessary coupling between the tuned primary circuit and the tuned secondary circuit.

The tube 8 has fiber plates l4 and I6 attached to its ends for supporting and centering it in a shielding can H. The tube 9 has fiber plates I8 and [9 attached to itsends for supporting and centering it in the shielding can H.

The primary and secondary circuits are tuned for the proper adjustment ofthe band. pass filter by varying the inductance of the primary coil I l by means of a suitable magnetic core indicated at 2| and by varying the inductance of the secondary coil I2 by means of a suitable magnetic core indicated at 22. These cores may be of any suitable material, such as Magnetite. The magnetic cores 2| and 22 are attached to threaded rods 23 and 24, respectively, which are supported from the fiber plates l4 and I8. After the circuits have been properly tuned, the threaded rods are prevented from turning by means of U-shaped pieces 26 and 210i spring wire, which function as lock nuts. I

The coil supporting tubes 8 and 9 are mounted inside the shielding can H, which may be of aluminum or the like, this can having a partition 28 which shields the primary and secondary coils from each other.

A blocking condenser 29 and a grid resistor 3 I, which are connected as described hereinafter, are supported from the lower ends of the fiber tubes 8 and 9. The complete assembly of transformer coils and shielding can is mounted on the upper side of the sub-base 2 and adjacent to the electric discharge tube 3.

' In order to provide additional shielding between the grid terminal and the grid lead indicated at 30, which connects successive amplifier stages, a shield plate 35 of aluminum or the like is attached to the shielding can [1. The plate 35 extends above the top of the tube shield 6, although it is shown broken off in the drawmgs. l

The invention will be better understood by referring to the circuit diagram shown in Fig. 3. In this diagram, the portion of the amplifier enclosed by the broken line 32 is the amplifier stage illustrated in Figs. 1 and 2, like parts in the three figures being indicated by the'same reference numerals. The dotted rectangle 33 indicates one-half of the shielding can I1, while the dotted rectangle 34 indicates the other half of the can H.

In Fig. 3, we have shown two amplifier tubes3 and 36 of an intermediate frequency amplifier, these amplifier tubes being coupled by means of a band pass filter constructed in accordance with a preferred embodiment of our invention. The amplifier tube 3, which is preferably'of the pentode type, such as a EDS, comprises an indirectly heated cathode 31, a control grid 38, a screen grid 39, a suppressor grid 4|, and a plate 42. The second amplifier tube may be of the same type, comprising an indirectly heated cathode 43, a control grid 44, a screen grid 46, a suppressor grid 41, and a plate 43. It is assumed, for the purpose of illustration, that the amplifier stage which is being described, and the amplifier stages preceding and succeeding it, are of the same type.

As illustrated in the specific circuit shown in Fig. 3, a modulated intermediate frequency signal is supplied to the control grid 38 of the tube 3, through a blocking condenser 49 and a tuned secondary winding A suitable bias potential is supplied to the control grid 38 through a grid resistor 52. In the specific amplifier illustrated, the screen grid 39 and the suppressor grid II of the amplifier tube 3 are connected together for the purpose of increasing the mutual conductance of the tube since it generally is difilcult to obtain the necessary amplification at the wide band width desired in a television reoeiver. Positive voltage is supplied to the suppressor grid ll and screen grid 39 through a filter resistor 53, while the usual filter condenser 54 is connected between the high voltage end of the resistor 53 and ground. It has been found that it is desirable to connect a resistor 56 of fairly low resistance value, such as 33 ohms, between the screen grid 39 and the suppressor grid II for the purpose of suppressing parasitic oscillations in the screen grid circuit. Also, it is desirable to provide a bypass condenser 53 across the terminals of the heater filament for cathode 31.

A suitable positive voltage is supplied to the plate ofthe amplifier tube 42 through a filter resistor 57, the coupling coil I3 and the primary coil I I, all connected in series. A suitable filter condenser 58 is connected between the upper end of the filter resistor 51 and ground. It will be noted that the output capacity of the amplifier tube 3, indicated at 59, is in series with the primary coil II to form a series resonant circuit. This resonant circuit, which is referred to as the tuned primary circuit, may be traced from the lower end of the coupling coil I3 through the primary coil II and through the output capacity 59 to ground, and from ground through the low impedance filter condenser 58 back to the lower end of the coupling coil I3. As it is usually necessary to provide damping in either the primary circuit or the secondary circuit for a band pass filter of this type, a damping resistor 3| is connected across the primary coil II and coupling coil I3.

The junction point of the coupling coil I3 and primary coil II is connected through the blocking condenser 29 and through the secondary coil l2 to the control grid 44 of the second amplifier tube 35. A suitable bias potential is applied to the control grid 64 through the grid resistor 3| which, inapreferred modification. is connected to the low intermediate frequency potential of the secondary coil l2. A suitable filter condenser 62 is connected between the low potential end of the grid resistor 3| and ground.

The secondary coil I2 is included in a series resonant circuit which is referred to as the tuned secondary circuit. This circuit may be traced from the lower end of the coupling coil I3, through the coupling coil l3 and the blocking condenser 29, the secondary coil I2, through the input capacity of the tube, indicated at 33, to ground, and through ground and the filter condenser 58 back to the lower end of the coupling coil I3.

Referring again to Figs. 1 and 2, the filter resistors 53 and 51 and the filter condensers 54, 58 and 62'are preferably mounted on a panel 63 on the under side of the sub-base 2, and supported therefrom. The resistor 56 is supported on the under side of the tube socket I by the connections to the screen grid and suppressor grid terminals. The by-pass condenser 50 for the heater filament of the cathode 31 is likewise supported by the connections to the filament terminals of the tube socket.

It will be noted that the second amplifier tube also has its screen grid connected to its suppressor grid through a resistor (i l of low resistance value for the reasons given in connection with amplifier tube 3.

In adjusting the band pass filter to obtain the desired selectivity curve, the same practice is followed as in the case of two inductively coupled tuned transformer windings. The primary and secondary circuits are tuned to substantially the same frequency and the coupling between them is so adjusted by winding the correct number of turns on coil I3 that the selectivity curve is fiat-topped.

One of the main advantages of our improved filter circuit is that the coupling between the two tuned circuits is determined solely by the coupling coil I3. It may be determined very exactly just how many turns are required on this coupling coil to provide the desired coupling between circuits. As a result, these filter units maybe manufactured by mass production methods and the coupling will always be of the desired uniform value. This avoids a rather serious difficulty that is present when the two circuits are coupled inductively, since the spacing between the inductively coupled coils is rather critical.

It will be noted that in the circuit shown in Fig. 3 the plate voltage fortube 3 is supplied through the coupling coil l3 and the primary coil II in series, rather than through a plate resistor. Usually this is a decided advantage, as it avoids the rather large voltage drop that is present in a plate resistor when the amplifier tube draws a substantial amount of plate current.

It will be understood, of course, that additional capacity may be provided by means of trimmer condensers or the like, for tuning the primary and secondary circuits in the event that the tube capacities are insufficient. As a rule, such additional capacity is unnecessary and undesirable at the high intermediate frequencies employed for television reception.

It will also be understood that the damping resistor 6| may be removed from the primary circuit and suitable damping provided in the secondary circuit. For example, the grid resistor 3| maybe connected tothe high potential or grid end of the secondarycoil I2, whereby it is, in a effect, connected across the coupling coil l3 and filter elements than is possible when .the grid resistor is connected to the high potential end of. this coil. It is obvious that the lower the potential of the point to which the resistor 3| is connected, the less filtering is there required of the filter condenser 62. It may be noted that the impedance of the grid resistor 3! is very high compared with the impedance of the coupling coil l3, the impedance of the one being about one-half megohm and the impedance of the other being about 250 ohms at the I. F. carrier frequency, whereby the grid resistor 3| has but little effect upon the tuned secondary circuit.

We claim as our invention:

1. An intermediate frequency amplifier unit comprising a sub-base having an electric discharge tube socket mounted thereon for supporting an electric discharge tube on the upper side thereof, a pair of coil tubes of insulating material mounted vertically on the upper side of said sub-base and positioned adjacent to said electric discharge tube, a primary coil and a coupling coil wound on one of said coil tubes, a secondary coil wound on the other of said coil tubes, a movable magnetic core in each of said coil tubes, one of said cores being positioned adjacent to its associated primary coil and removed from said coupling coil, the other of said cores being positioned adjacent to its associated secondary coil, connections for supplying voltage to the plate of said electric discharge tube through said coupling coil and said primary coil, a blocking condenser connected between one end of said secondary coil and the junction point of. said primary coil and said coupling coil, a lead connected to the other end of said secondary coil whereby it may be connected to a control electrode of a second discharge tube, and a resistor connected to one end of said secondary coil whereby a bias potential may be supplied to said control electrode through said resistor, the circuits including said primary and secondary coils being so tuned and said circuits being so coupled through said coupling coil as to provide a filter circuit having a band-pass characteristic. I

'2. A high frequency amplifier comprising an amplifier tube having a plate electrode and output capacity, a second amplifier tube having a control electrode and input capacity, a primary coil, a blocking condenser, and a secondary coil all connected in series between said plate electrode and said control electrode, a coupling coil connected to the junction point of said primary coil and said blocking condenser, means for supplying plate voltage to said plate electrode through said coupling coil and said primary coil, a grid resistor connected to one end of said secondary coil whereby a biasing voltage may be supplied therethrough to said control electrode, means for tuning the circuit including said primary coil, said coupling coil and said output capacity to a certain frequency, and means for tuning the circuit including said secondary coil, said coupling coil and said input capacity to substantially said certain frequency, said coupling being such as to give said amplifier substantially a fiat-topped characteristic, said primary and secondary coils being so positioned with respect to each other that substantially the only coupling between them is provided by said coupling coil.

3. A'high frequency amplifier comprising an amplifier tube having a plate electrode and output capacity, a second amplifier tube havinga control electrode and input capacity, a primary coil, a blocking condensenand a secondary coil all connected in series between said plate elec-.

trode and said control electrode, a coupling coil connected to the junction point of said primary coil and said blocking condenser, means for supplying plate voltage to said plate electrode through said coupling coil and said primary coil, a grid resistor connected to one end of said secondary coil whereby a biasing voltage may be supplied therethrough to said control electrode, magnetic means for varying the inductance of, said primary coil and for tuning the circuit including said primary coil, said coupling coil and.

said output capacity to a certain frequency, and magnetic. means for varying the inductance of said secondary coil and for tuning the circuit including said secondary coil, said coupling coil and said input capacity to substantially said certain frequency, said coupling being such asto give said amplifier substantialy a fiat-topped.

characteristic, each of said magnetic means comprising a core of magnetic material, one of said cores being positioned adjacent to said primary coil and the other of said cores being positioned adjacent to said secondary coil, and both of said cores being substantially removed from said coupling coil whereby an adjustment of the position of said cores will have substantially no effect upon the inductance of said coupling coil, said.

primary and secondary coils being so positioned with respect to each other that substantially the only coupling between them is provided by said coupling coil.

4. A high frequency amplifier comprising an amplifier tube having a plate electrode and output capacity, control electrode and input capacity, aprimary coil, a blocking condenser, and 'a secondary coil all connected in series between said plate electrode and said control electrode, a coupling coil connected to the junction point of said primary coil and said blocking condenser, means for supplying plate voltage to said plate electrode through said coupling coil and said primary coil, a grid resistor connected at one end to the end of said secondary coil which is adjacent to said blocking condenser whereby a biasing voltage may be supplied therethrough to said control electrode, a filter condenser connected between the other end of said grid resistor and ground, magnetic means for varying the inductance of said primary coil and for tuning the circuit including said primary coil, said coupling coil and said output capacity to a certain frequency, and magnetic means for varying the inductance of said secondary coil and for tuning the circuit including said secondary coil, said coupling coil and said input capacity to substantially said certain frequency, said coupling being such as to give said amplifier substantially a fiat-topped characteristic; I HORACE C. ALLEN. GUSTAVE L. GRUNDMANN.

a second amplifier tube having a 

